1. Field of the Invention
The present invention generally relates to a corrugated fin type heat exchanger for heating air by heat exchanging hot water with the air, and is preferably applied to a corrugated fin type heat exchanger used in an automotive air conditioner in which hot water flow quantity widely varies.
2. Related Art
In a vehicle, as illustrated in FIG. 1, a heat exchanger 2 for heating is installed in a cooling water (hot water) circuit of an engine 1 for running the vehicle. Hot water is circulated into the heat exchanger 2 by a water pump 3 driven by the engine 1, and the flow quantity of the hot water flowing from a flow quantity control valve 4 into the heat exchange 2 is controlled to adjust the temperature of the air flow of the heat exchanger 2.
Engine cooling water is circulated into a radiator 6 by the water pump 3 through a thermostat 5 to cool the engine cooling water within the radiator 6. The thermostat is a well-known device, in which a valve opens when the cooling water temperature rises to or exceeds a predetermined temperature, thereby the cooling water flowing into the radiator 6.
The reference numeral 7 denotes a bypass circuit for the engine cooling water; numeral 8 denotes a radiator side circuit; and numeral 9 denotes a heater side circuit. The water pump 3 circulates the cooling water through all of these circuits 7, 8 and 9.
However, as the water pump 3 is driven by the engine 1, a rotational speed of the water pump 3 largely varies according to the rotational speed of the engine 1, i.e., the vehicle speed, and thereby flow quantity of the hot water into the heat exchanger 2 largely varies.
As a result of such large variation in the flow quantity of the hot water into the exchanger 2, when the vehicle is running at a low speed (when the hot water flow quantity is small), as illustrated in FIG. 2, there is a problem in that the heat radiation performance of the heat exchanger 2 is extremely deteriorated.
In FIG. 2, the ordinate represents the heat radiation performance Q of the heat exchanger 2, the abscissa represents the flow quantity Vw of the hot water into the heat exchanger 2. As can be seen from FIG. 2, the hot water flow quantity is 16 lit/min when the vehicle is running at 60 km/h, and the hot water flow quantity is 4 lit/min when the vehicle is in idling. As the hot water flow quantity decreases, the heat radiation performance when the vehicle is idling falls by 22% as compared to when the vehicle is running at 60 km/h. As a result, heat generation is reduced.
Particularly when the vehicle is running on urban streets, the vehicle is subjected to frequent starts and stops due to traffic signals. Therefore, whenever the vehicle engine is idling, there is insufficient heat for the passengers.
The inventors of the present invention have studied the cause of such deterioration of the heat radiation performance from various points of view and have determined the following.
As illustrated in FIG. 3, the heat exchanger 2 includes a plurality of flat tubes 2a arranged in parallel with the air flow direction. These flat tubes 2a are individually disposed in a single row in the air flow direction. Corrugated fins 2b are disposed between each pair of flat tubes 2a, thereby configuring a corrugated type heat exchanger. The reference numeral 2c denotes a core portion which is composed of the flat tubes 2a and the corrugated fins 2b.
In FIG. 4, the ordinate represents water side heat transfer rate .alpha.w of the flat tube 2a, and the abscissa represents the Reynold's number Re and hot water flow quantity Vw of the hot water passages formed with the flat tubes 2a.
As understood from FIG. 4, the Reynold's number is within range of 500-2200 when the hot water flowing into the heat exchanger 2 is within a predetermined range (16 lit/min when the vehicle is running at 60 km/h, and 4 lit/min when the vehicle is idling), and the heat exchanger 2 is operated to the extent from the laminar region to a transition flow region. For this reason, the water side heat transfer rate .alpha.w largely varies in accordance with the variation of the hot water flow quantity. As a result, it turned out that the water side heat transfer rate .alpha.w largely falls within the low flow quantity region, thereby causing the deterioration of the heat radiation performance when the vehicle is idling.
FIG. 4 illustrates the results of an experiment in which normal tubes with no dimples (concave and convex portion) for facilitating the turbulence of the hot water on the inner surfaces were used as the flat tubes 2a.
For improving the water side heat transfer rate .alpha.w, in general, the turbulence of the hot water within the tubes is often facilitated. Concretely, it has been proposed that a turbulence generator for facilitating turbulence is inserted into the tubes, or dimples are formed on the inner surfaces of the tubes to facilitate turbulence.
Therefore, the inventors of the present invention have measured the water side heat transfer rate .alpha.w by using the flat tubes 2a with dimples for facilitating turbulence. As a result, as illustrated in FIG. 5, the flat tube with dimples could generally improve the water side heat transfer rate .alpha.w as compared to the normal tube, and the Reynold's number Re of the dimple tube in the transition region from laminar to turbulence decreased from 1400 with the normal tube to 1000.
However, the large variation in the water side heat transfer rate .alpha.w according to the hot water flow quantity still remained even when the flat tube with dimples is used. Therefore, even when a technique for facilitating a turbulence such as the flat tubes with dimples is used, it is not possible to solve the problem of reduced heat radiation performance when the hot water flow quantity is small (when the vehicle is running at a low speed).